1. Field of the Invention
The present invention relates to a backlight device using a linear light source such as a cold cathode tube and a point light source such as an LED, and a display device using the same.
2. Description of the Related Art
In recent years, as a flat panel display having features of a smaller thickness and a smaller weight compared with a conventional cathode ray tube, a liquid crystal display device, for example, has been used widely for a liquid crystal television, a monitor, a mobile phone and the like. Such a liquid crystal display device includes a backlight device that emits light and a liquid crystal panel that serves as a shutter with respect to light from a light source provided in the backlight device, thereby displaying a desired image.
In the above-noted backlight device, a linear light source such as a cold cathode tube or a point light source such as a light-emitting diode (LED) is used such that light from these light sources is turned to planar light having a substantially uniform brightness and irradiated onto the liquid crystal panel.
Further, the backlight devices are roughly classified into those of a direct light type and those of an edge light type according to the light source arrangement relative to the liquid crystal panel. In other words, in the direct light-type backlight device, the light source is arranged on a lower side (a non-display surface side) of the liquid crystal panel, and a diffusing plate is interposed between the light source and the liquid crystal panel so as to allow the above-noted planar light to enter the liquid crystal panel.
On the other hand, the edge light-type backlight device has a configuration in which the light source is arranged on a lateral side of the liquid crystal panel, and light from this light source is introduced to a light guide plate that is disposed on the lower side of the liquid crystal panel. Then, in the edge light-type backlight device, the light from the light source is turned to the above-noted planar light by the light guide plate and irradiated from a light emitting surface of the light guide plate arranged in opposition to the non-display surface of the liquid crystal panel.
Also, in a conventional backlight device, it has been suggested to use a plurality of auxiliary light guide plates in addition to a main light guide plate that is arranged in opposition to the liquid crystal panel, as described in JP 2005-250020 A, for example. More specifically, in this conventional backlight device, two auxiliary light guide plates are stacked sequentially on the above-mentioned main light guide plate on the side of an opposed surface that is opposed to a light emitting surface of the main light guide plate, and light from a plurality of LEDs that are aligned is allowed to enter inner portions of the respective auxiliary light guide plates. Then, in this conventional backlight device, the light emitted from each of the auxiliary light guide plates is introduced to the main light guide plate by using a reflecting mirror, thereby increasing the number of the LEDs to be disposed so as to achieve higher brightness while reducing the device size.
However, in the conventional backlight device as described above, since the light that has left each of the auxiliary light guide plates is led to the main light guide plate by being reflected by the reflecting mirror (reflector), a utilization efficiency of the light from the LED (light source) has lowered in some cases depending on a light reflection angle at the reflecting mirror, which is defined by a radius of curvature, a reflectance, etc. of an inner surface of the reflecting mirror, and the number of the auxiliary light guide plates to be disposed. In other words, in the auxiliary light guide plates that are stacked in two tiers on the main light guide plate in the conventional backlight device, there have been some cases where the light that has left the auxiliary light guide plate in the first tier enters the inner portion of the auxiliary light guide plate in the second tier via the reflecting mirror, or conversely, the light that has left the auxiliary light guide plate in the second tier enters the inner portion of the auxiliary light guide plate in the first tier via the reflecting mirror. As a result, in the conventional backlight device, the light from each of the auxiliary light guide plates has not been led directly to the main light guide plate but has been absorbed by the inner portion of the auxiliary light guide plate to which the light has led erroneously via the reflecting mirror, resulting in the reduction of the utilization efficiency of light from the LED.